chapter  5
Extraction and Analysis of Lipids
Pages 32

I. Introduction.......................................................................................................................... 126 II. Extraction of Lipids from Foods and Biological Materials................................................. 126

A. Sample Preparation ...................................................................................................... 127 B. Pretreatments ................................................................................................................ 128

1. Drying ................................................................................................................... 128 2. Particle Size Reduction......................................................................................... 128 3. Acid=Alkali Hydrolysis......................................................................................... 128

C. Lipid Extraction with Solvents .................................................................................... 129 1. Properties of Solvents and Their Mode of Extraction.......................................... 129 2. Extraction Methods with Single Organic Solvent ................................................ 129 3. Methods Using Organic Solvent Combination..................................................... 130 4. Methods Using Nonorganic Solvents ................................................................... 132

D. Lipid Extraction without Solvents ............................................................................... 133 1. Acid Digestion Methods ....................................................................................... 133 2. Detergent Method ................................................................................................. 133 3. Physical Methods.................................................................................................. 133

E. Removal of Nonlipid Contaminants from Lipid Extracts and Other Practical Considerations .............................................................................................................. 133

III. Indirect Methods of Total Lipid Determination .................................................................. 134 A. Density Measurement .................................................................................................. 135 B. Dielectric Method......................................................................................................... 135 C. Near-Infrared Spectroscopy ......................................................................................... 135 D. Low-Resolution Nuclear Magnetic Resonance Spectroscopy..................................... 135 E. Turbidimetric=Colorimetric Methods........................................................................... 135 F. Ultrasonic Method........................................................................................................ 136 G. X-Ray Absorption........................................................................................................ 136

IV. Analysis of Lipid Extracts ................................................................................................... 136 A. Bulk Oil Properties ...................................................................................................... 136

1. Degree of Unsaturation......................................................................................... 136 2. Free Fatty Acid (FFA) Content ............................................................................ 137 3. Oxidative Stability and Oxidation Products ......................................................... 137 4. Refractive Index.................................................................................................... 137 5. Saponification Value............................................................................................. 137 6. Solid-Fat Index ..................................................................................................... 138

B. Chromatographic Procedures for Lipid Characterization ............................................ 138 1. Column Chromatography ..................................................................................... 138 2. Gas Chromatography ............................................................................................ 139

Chromatography 140 4. Supercritical Fluid Chromatography..................................................................... 141 5. Thin-Layer Chromatography ................................................................................ 142

C. Spectroscopic Methods of Lipid Analysis................................................................... 144 1. UV-Visible Spectroscopy ..................................................................................... 144 2. Infrared Absorption Spectroscopy ........................................................................ 145 3. Nuclear Magnetic Resonance Spectroscopy......................................................... 146 4. Mass Spectrometry................................................................................................ 147

D. Enzymatic Methods ..................................................................................................... 148 E. Immunochemical Methods........................................................................................... 149

V. Summary.............................................................................................................................. 149 References ..................................................................................................................................... 149

Lipids are among the major components of food of plant and animal origin. There is no precise definition available for the term lipid; however, it usually includes a broad category of compounds that have some common properties and compositional similarities. Lipids are materials that are sparingly soluble or insoluble in water, but soluble in selected organic solvents such as benzene, chloroform, diethyl ether, hexane, and methanol. Together with carbohydrates and proteins, lipids constitute the principal structural components of tissues. However, the common and unique features of lipids relate to their solubility rather than their structural characteristics [1]. Many classification systems have been proposed for lipids. From the nutrition point of view, according to the National Academy of Sciences’ report on nutrition labeling, fats and oils are defined as the complex organic molecules that are formed by combining three fatty acid molecules with one molecule of glycerol [2]. As indicated in Table 5.1 [3-5], lipids are generally classified as simple and compound (complex) or derived lipids according to the Bloor [3] classification.